1. Field of the Invention
The invention relates to a device and a method that diagnose the presence/absence of an abnormality of an air-fuel ratio sensor that detects the air-fuel ratio of exhaust gas of an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine equipped with an exhaust gas purification system that uses a catalyst, it is indispensable to control the mixing proportion between air and fuel in a mixture burned in the internal combustion engine, that is, the air-fuel ratio, in order to effectively remove harmful components of exhaust gas through the use of the catalyst. In order to perform the control of the air-fuel ratio, an air-fuel ratio sensor that detects the air-fuel ratio on the basis of the concentration of a specific component of exhaust gas is provided on an exhaust passageway of the internal combustion engine, and feedback control is performed so as to bring the detected air-fuel ratio closer to a predetermined target air-fuel ratio.
If the air-fuel ratio sensor has an abnormality such as degradation, a failure, etc., it becomes impossible to perform accurate air-fuel ratio feedback control, and therefore exhaust gas emissions deteriorate. Therefore, diagnosis of abnormality regarding the air-fuel ratio sensor has been conventionally conducted. In particular, in the case of an engine mounted in a motor vehicle, laws and regulations of various countries require that abnormality regarding the air-fuel ratio sensor mounted in the vehicle (on board) be detected in order to prevent the vehicle from running while emitting deteriorated exhaust gas.
Japanese Patent Application Publication No. 2005-30358 (JP-A-2005-30358) discloses an abnormality detection device for an air-fuel ratio sensor that periodically increases and decreases the air-fuel ratio through an open-loop control and that detects an abnormality of the air-fuel ratio sensor on the basis of the length and area of the locus of the output of the air-fuel ratio sensor that increases and decreases corresponding to the periodical increase and decrease in the air-fuel ratio. Besides, Japanese Patent Application Publication No. 2004-68602 (JP-A-2004-68602) discloses an air-fuel ratio control device that serially identifies (e.g., determines, or estimates) a plant model that represents a detection delay characteristic of the air-fuel ratio sensor, and sets a control gain in an air-fuel ratio feedback control through the use of parameters of the identified plant model. In this related art, the recursive identification is stopped when the diagnosis regarding the response degradation of the air-fuel ratio sensor is performed during the feedback control.
However, although the technology described in Japanese Patent Application Publication No. 2005-30358 (JP-A-2005-30358) is able to discriminate whether the air-fuel ratio sensor itself is normal or abnormal, the technology is not able to discriminate which of the characteristics of the air-fuel ratio sensor is normal or abnormal. Specifically, although the air-fuel ratio sensor has a plurality of characteristics, the technology described in Japanese Patent Application Publication No. 2005-30358 (JP-A-2005-30358) cannot discriminate which of the characteristics is abnormal.
Besides, the technology described in Japanese Patent Application Publication No. 2004-8602 (JP-A-2004-68602) carries out a diagnosis only about degradation of the response rate among the various characteristics of the air-fuel ratio sensor. However, although the air-fuel ratio sensor has other characteristics, abnormality regarding any of these other characteristics cannot be discriminated. In the first place, the technology described in Japanese Patent Application Publication No. 2004-68602 (JP-A-2004-68602) relates to the air-fuel ratio control, and is not specialized for the abnormality diagnosis on the air-fuel ratio sensor. Hence, the parameters of the serially identified plant models are used to set a control gain in the air-fuel ratio feedback control, and the serial estimation is stopped at the time of diagnosis about the response delay of the air-fuel ratio sensor.